Metal surface scale conditioning

ABSTRACT

Methods and systems are provided for treating oxide scale on the surface of a metal object. In one embodiment, a system temperature control apparatus controls the temperature of metal object&#39;s surface to an application temperature below the Leidenfrost temperature point of an alkali metal hydroxide aqueous conditioning solution. An application apparatus wets the metal object&#39;s surface at the controlled temperature with a thin layer of the solution that engages the oxide scale, and a heating apparatus heats the wetted surface to a final conditioning temperature above a melting point of the alkali metal hydroxide by an additional value selected to effect conditioning of the oxide scale at a reasonable but not excessive rate by the melting alkali metal hydroxide reacting with the oxide scale. The system terminates additional conditioning to prevent creation of additional oxide scale beyond the conditioned depth.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation, and claims priority, of a U.S.provisional patent application by Malloy et al for METAL SURFACE SCALECONDITIONING, filed in the U.S. Patent and Trademark Office on Jan. 11,2010 and assigned Ser. No. 61/293,821, confirmation number 6931.

FIELD OF THE INVENTION

This invention relates generally to conditioning of oxide or scale onmetal surfaces.

BACKGROUND OF THE INVENTION

The conditioning of oxide surfaces or scale on metal surfaces, sometimesreferred to as descaling, is desired with respect to the production ofstainless steel and superalloy metal strips. While our discussionfocuses primarily on metals in strip form, the applicability and valueof our invention may be useful for conditioning oxide surfaces or scalein various shapes, geometries, or assemblies other than metal strip; itis not our intention to limit the benefit to only metal strip. Stainlesssteels are ferrous alloys containing more than about 10% chromium forthe purpose of enhancing corrosion and oxidation resistance, and mayalso contain nickel, molybdenum, silicon, manganese, aluminum, carbideformers and other elements. Families of superalloys may contain nickelor cobalt as the predominant base element, and incorporate more exoticalloying elements, such as tungsten, titanium, niobium, and otherelements. All of these base elements and additive elements have a highaffinity for oxygen at high temperatures and form tenacious andchemically stable oxides which complicate their subsequent removal whichis required prior to additional processing or sale.

Prior art descaling techniques for some grades of low alloy steels withvery light scale include pickling of steel strip in mineral acid, suchas sulfuric acid, hydrochloric acid, hydrofluoric acid, nitric acid, ormixtures thereof. However, often a mere acid pickle is insufficient intreating higher alloy steel strips. Conditioning of the scale beforeacid pickling may be required. Typical compositions used for scaleconditioning are caustic mixtures of alkali metal hydroxides and alkalimetal nitrates with various other additives such as alkali halidescarbonates, and/or other oxidizing agents, often referred to asdescaling or scale conditioning salts. A conventional technique forusing such compositions is in a bath of fused anhydrous salt in a vesselat elevated temperatures, e.g. 427° C. (800° F.) to 538° C. (1000° F.),in which a metal object is first immersed, followed by a water rinse andacid pickle.

BRIEF SUMMARY OF THE INVENTION

Methods and systems are provided for treating oxide scale on the surfaceof a metal object. In one embodiment, a system (100) includes atemperature control apparatus (105) that controls the temperature ofmetal object's (106) surface (112) to an application temperature belowthe Leidenfrost temperature point of an aqueous conditioning solutioncomprising an alkali metal hydroxide, wherein the metal object's surfacehas an oxide scale having an initial depth from the metal object'ssurface. An application apparatus (108) wets the metal object's surfaceat the controlled temperature with a thin layer (111) of the aqueousconditioning solution which engages the oxide scale. A heating apparatus(113) heats the wetted metal object surface to a final conditioningtemperature that is above a melting point of the alkali metal hydroxidein an anhydrous form by an additional value selected to effectconditioning of the oxide scale on the metal surface at a reasonable butnot excessive rate, the heated wetted metal object surface therebyevaporating water in the aqueous conditioning solution and melting thealkali metal hydroxide in the anhydrous form on the metal object'ssurface, wherein the melting alkali metal hydroxide reacts with theengaged oxide scale and reduces the oxide scale to a conditioned depthfrom the metal object's surface that is less than the initial depth. Thesystem further terminates additional conditioning of the metal object'ssurface beyond the conditioned depth, the terminating preventing acreation of an additional oxide scale beyond the conditioned depth fromthe metal object's surface.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a graphic illustration of iron, chromium, nickel and oxygen asnormalized weight percentages as a function of distance in Angstromsfrom the surface of an exemplary annealed type 304 stainless steelsample.

FIG. 2 is a graphic illustration of the normalized weight percentages ofiron, chromium, nickel and oxygen in the type 304 stainless steel sampleof FIG. 1 after immersion in a conventional, prior art salt-bath for aconventional time frame.

FIG. 3 is a graphic illustration of the normalized weight percentages ofiron, chromium, nickel and oxygen in the type 304 stainless steel sampleof FIG. 1 after immersion in a conventional, prior art salt-bath for anextended time frame.

FIG. 4 is a graphic illustration of the normalized weight percentages ofiron, chromium, nickel and oxygen in the type 304 stainless steel sampleof FIG. 1 after conditioning according to the present invention.

FIG. 5 is a diagrammatic view of a process for scale conditioningaccording to the present invention.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

There are a variety of drawbacks to immersion salt bath techniques. Thesalt bath has to be maintained at elevated temperatures, which may beenergy intensive. Fused caustic baths requiring submerged rolls may bedifficult to maintain and may cause marring of the surface of the stripbeing descaled. There are “drag-out” problems and hazards with respectto treating strip steel with the heated fused composition: as the stripexits from a pot of fused composition, it carries a certain amount ofthe heated, fused composition with it, especially at high strip speeds.Efforts to incorporate metal wiping rolls to reduce this chemicaldrag-out from the bath may introduce their own set of processcomplications including scratching or marring the fine metal surfaces.The long-term high temperature exposure that these fused bathcompositions are subjected to limit the compounds that may beincorporated into the working bath, further restricting processflexibility.

The use of immersion-type salt bath conditioning may also result inover-conditioning of the metal surface with excess oxide formation aswell as other detrimental effects. FIG. 1 provides a graphicillustration of Scanning Auger Microprobe (SAM) profiles of iron (Fe)12, chromium (Cr) 14, nickel (Ni) 16 and oxygen 18 representingnormalized weight percentages (the vertical axis 20) as a function ofdistance in Angstroms (Å) from the surface (the horizontal axis 22) of atype 304 stainless steel sample annealed in a gas-fired furnace in anoxygenated atmosphere with 3% excess oxygen at a temperature of 1925° F.(1052° C.) for 120 seconds. The oxygen profile 18 is indicative ofrelative amounts of chromium oxide and iron oxide; as the oxygen levelsdiminish, so too do corresponding amounts of chromium oxide and ironoxide, and thus correspondingly increasing amounts of chromium and ironinstead of chromium oxide and iron oxide. The surface of the annealedtype 304 stainless steel sample (the region ranging from zero to 2000 Åalong axis 22) is shown to be composed of primarily chromium oxide, withdeeper regions progressively stabilizing until at from about 8000 Å to10000 Å where the sample has a composition of about 18% Cr and 8% Ni,the typical and expected composition of type 304 stainless steel andthus beyond an extent needed for scale conditioning, and further whereinremoval of any excess oxide formation may result in undesired surfaceeffects as well as unnecessary and costly additional pickling processes.

FIG. 2 provides a graphic illustration of SAM elemental depth profilesof the normalized weight percentages of iron 42, chromium 44, nickel 46and oxygen 48 in a 10.16 centimeter (4 inch)×15.24 centimeter (6 inch)panel of 0.635 millimeter (0.025 inch) gage type 304 annealed stainlesssteel type 304 (18/8 chrome-nickel) after a conventional, prior artsalt-bath conditioning treatment. The salt bath was an essentiallyanhydrous composition (i.e. it does not comprise enough water to reactwith the composition or a metal object surface submerged therein)comprising about 12% by weight sodium nitrate, about 10% by weightsodium chloride, about 15% by weight potassium hydroxide and about 63%by weight sodium hydroxide. This salt bath composition is taught in U.S.Pat. No. 3,260,619 issued to Shoemaker et al on Jul. 12, 1966, theentire disclosure of which is hereby incorporated, though it will beunderstood that alternative salt bath embodiments taught therein andelsewhere may also be used for conventional immersion salt bathconditioning. The profiles 42, 44, 46 and 48 were obtained afterimmersing the sample for a conventional, prior art time period of 30seconds in the molten salt bath heated to an operating temperature of850° F. (454° C.), the sample then removed from the bath and salt bathcomposition still adhering to the sample allowed to drip off for a fewseconds, and the sample then promptly plunged into a pail of room(ambient) temperature tap water and subsequently air dried.

In contrast to the profile illustrated in FIG. 1, FIG. 2 shows that theoriginal surface chromium oxide levels have been almost completelyremoved, with only iron oxide remaining and a residual conditioned scaleoccurring from about 4000 to about 5000 Å, where after (from 5000 Å anddeeper from the surface) the stainless steel sample compositionstabilizes. Of further interest is a “shoulder” region 50 on the ironprofile 42 from about zero to about 2000 Å, wherein the normalizedweight percentage of iron generally oscillates between about 55% and 58%as the depth increases over this range, until starting to progressivelyclimb after 2000 Å in depth. This oscillation suggests that excessiveconditioning processes are occurring, unnecessarily increasing theamount of conditioned oxide scale that must be subsequently removed, asfurther discussed below and which becomes even more apparent withreference to FIG. 3.

Reaction to the scale on a strip generally occurs to completion quicklyupon immersion in such baths; however, the logistics of strip processinggenerally dictate that the strip remains submerged in the bath wellafter optimal conditioning has already been obtained; since theconditioning reaction progresses so far so rapidly, over-conditioningnecessarily occurs before the conditioned strip leaves an immersionbath, thus obviating any opportunity for timely quenching to preventover-conditioning. FIG. 3 is a graphic illustration of SAM profiles ofnormalized weight percentages of iron 52, chromium 56, nickel 58 andoxygen 54 in a 0.635 millimeter (0.025 inch) gage, 10.16 centimeter (4inch)×15.24 centimeter (6 inch) panel of the type 304 annealed stainlesssteel of FIG. 1 after immersion in the prior art salt-bath of FIG. 2 foran extended time frame, namely for about 120 seconds. As before withrespect to FIG. 2, the sample was then removed from the bath, salt bathcomposition still adhering to the sample allowed to drip off for a fewseconds, and then the sample was promptly plunged into a pail of room(ambient) temperature tap water and subsequently air dried. FIG. 3clearly shows the deleterious effects of long immersion timeconditioning, which may happen when a continuous process strip lineincorporating salt bath conditioning stops or slows down and a stainlesssteel strip is left exposed to a molten immersion process for a longtime, e.g. 60, or 120 seconds or even longer. The iron profile 52 inview of the oxygen profile 54 indicates that iron oxide occurs at asteady and unacceptably high level over the length of the Auger scandata, to over 10000 Å of depth from the surface, with chromium 56reduced to almost half of its native, desired concentration. It isbelieved that this occurs because the iron oxide does not dissolve inthe molten salt (further indicated in the small shoulder region 50 ofFIG. 2), but instead the alkali chromate does, resulting in unacceptablediminishment of the chromium deep into the metal surface.

FIG. 4 provides SAM profiles of iron 62, chromium 64, nickel 66 andoxygen 68 for another 0.635 millimeter (0.025 inch) gage, 10.16centimeter (4 inch)×15.24 centimeter (6 inch) panel of the type 304annealed stainless steel of FIG. 1 as conditioned according to thepresent invention. An alkaline aqueous liquid was applied thinly to theannealed type 304 stainless at an ambient temperature, the coated samplethen heated in a horizontal orientation in an electric oven pre-heatedto about 800° F. (427° C.) for about 120 seconds, wherein the panel wasbrought to final treatment temperature of about 650° F. (343° C.), thesample then subsequently tap water rinsed within 30 seconds of reachingsaid treatment temperature and then air dried. FIG. 4 shows a clearcontrast to and improvement over the results profile of a conventionaltime and temperature immersion treatment depicted in FIG. 2. It will beunderstood by one skilled in the art that the ultimate goal inconditioning annealed stainless steel is to remove original surfacechrome oxide without adversely attacking the underlying metal (i.e.reducing the native chromium levels) and further without unnecessarilybuilding up new iron oxide that needs to be subsequently removed in thepickling section of the line. As FIG. 4 illustrates, a chrome oxide-freesurface is obtained relatively quicker and more efficiently relative todepth from the surface; the iron oxide shoulder 50 of FIG. 2 is avoided,the iron profile line 62 instead quickly and steadily climbing up to alevel content by the 2000 Å in depth, and further wherein the desiredcomposition of about 18% Cr 64 and 8% Ni 66 is also reached at 2000 Å indepth, an improvement of about 60% over the performance of theconvention immersion conditioning results depicted in FIG. 2 in reducingthe depth and extent of surface oxides.

The present invention is appropriate for practice with a wide variety ofmetals, illustratively but not exhaustively including stainless steelsand superalloys and their alloying elements such as manganese,molybdenum, titanium, etc. The invention is also applicable to reactingwith oxides of these and other alloying elements to form more easilyremoved species such as alkali manganates, molybdates, titanates, etc.,for example in the conditioning of titanium alloys, molybdenum alloys,etc., including as incorporated into other alloys as alloying agents.

According to the present invention, the time required to condition theoxide scale is virtually instantaneous once a final treatmenttemperature threshold is reached. For stainless steel, the finaltreatment temperature is believed to range from about 600° F. (315° C.)to about 650° F. (343° C.), and the selected or determined temperaturemay be dependent on material composition as well as dimensionalparameters. For example, the 650° F. (343° C.) was determined to be thefinal treatment temperature for the 0.635 millimeter (0.025 inch) gage,10.16 centimeter (4 inch)×15.24 centimeter (6 inch) panel of type 304annealed stainless steel of FIG. 1 by placing a panel coated on one sidewith an alkaline aqueous liquid according to the present invention in ahorizontal plane and then heating it (in one example from below with ahigh temperature hot air gun or by placing it on a resistance heatercoil). Once the coated surface reached the final treatment temperature,the 650° F. (343° C.) as determined by a fine diameter contactthermocouple, a central core area of the panel very quickly changed froman annealed color to an alkali chromate color characteristic ofconditioning, and wherein the conditioned area radially grew outward asthe critical temperature was reached in the peripheral areas of thepanel. Thus, achievement of the final treatment temperature andassociated complete oxide treatment according to the present inventionmay be determined by a visual examination of the annealed steel, forexample by looking for a glossy molten salt film and distinctive colorchange appearance. It will also be understood by one skilled in the artthat the time required for an annealed metal surface coated with analkaline aqueous liquid according to the present invention to reach thefinal treatment temperature is a function of a difference in temperature(delta T) between said final treatment temperature and the temperatureof the heating device (heat flux). By thinly coating the surface ofannealed panel to be treated, the present invention limits the chemicalsink present on the metal surface, and thus once the final treatmenttemperature is reached, little additional reaction takes place even ifthe metal object is held at the final treatment temperature for someadditional time.

Alkaline aqueous liquids according to the present invention compriseeutectic hydroxides, and fractional percentages of at least onesurfactant are included to help wet-out performance of the liquid andaid in maintaining thin coating dimensions. Some examples furtheroptionally comprise oxidizers to boost the oxidizing potential of theliquid, and compositions according to the present invention may becustom blended depending on the type and quantities of oxides that maybe present in the steel to be conditioned. One embodiment of an alkalineaqueous liquid according to the present invention used to obtain theconditioning illustrated in FIG. 4 is a eutectic blend of sodium andpotassium hydroxides at about a 33% by weight (more specifically, 18%potassium hydroxide and 12% sodium hydroxide), 3% by weight sodiumnitrate as an oxidizer boost, and 67% by weight water, and to which wasadded three drops each of Nonidet® SF-5 and Mirataine® ASC (NONIDET SF-5is a trademark of Air Products and Chemicals, Inc., in the United Statesor other countries; MIRATAINE is a trademark of Rhodia in the UnitedStates or other countries). Nonidet® SF-5 is a low foaming alkoxylatednonionic surfactant made from linear alcohol, and the chemical name ofMirataine® ASC is Cocamidopropyl Hydroxysultaine.

According to the present invention, the solubility of the reactionproducts, (e.g. alkali chromate) may be quickly reached in the thin andlight weight amount of the alkaline aqueous liquid incorporating thesurfactant. By keeping the coating layer thin, reactive chemicals in thealkaline aqueous liquid are substantially consumed immediately upon thecoated object reaching the final treatment temperature with littleresidual reactants remaining available for further oxide conditioning orother reactions with the treated metal object: thus, any time lag fromthe completion of conditioning to quenching or water rinsing isgenerally inconsequential to the performance of the process, and moreparticularly will not cause the over-conditioning harm to the metalsubstrate shown by the long-term immersion treatments as illustrated inFIG. 3. It is also apparent that by only requiring enough of thealkaline aqueous liquid to thinly coat an object to be treated, the needfor substantial additional quantities in order to form molten salt bathsis avoided, and thus the present invention enables greater material costand handling efficiencies relative to prior art immersion processes. Inaddition, since the chemical constituents of the descaling film onlyneed to be stable at temperature for a very short time, more novel orreactive chemical compounds may be employed than is possible intraditional immersion chemical formulations due to their need for longterm high temperature stability.

The present invention also provides superior energy efficienciesrelative to conventional immersion processes. In one aspect, it isnecessary to operate immersion process salt bath pots at temperatureswell above the final treatment temperature or range practiced by thepresent invention. The viscosities of the higher density anhydrous saltbath solutions appropriate for conventional immersion processes at theexemplary final treatment temperature range described above (from about600° F. (315° C.) to about 650° F. (343° C.)) are too high, prohibitingoperating the salt bath within this temperature range in order toprevent excessive salt drag-out, and requiring that the salt bath potsinstead be operated and held at much higher temperatures, such as fromabout 752° F. (400° C.) to about 932° F. (500° C.), in order to preventmaterial drag-out problems. It will also be noted that though otherprior art teaches conditioning metal objects by coating the objects withan alkali solution and then heating the coated objects in an annealingfurnace, these annealing processes require significantly highertemperatures, generally in excess of 1850° F. (1010° C.), and furtherfail to produce the efficient conditioned profile achieved in FIG. 4(i.e., reducing the depth of surface scale to and producing the desiredresultant composition of about 18% Cr and 8% Ni at 2000 Å in depth).

Moreover, the conventional immersion process provides an “infinite”chemical sink to continually accept reaction products and provide freshchemical reactants to a metal object being conditioned. For example, astainless steel strip being conditioned in a processing line exits animmersion bath at a temperature in excess of the optimum temperature forminimal excess oxide formation, inherently resulting in anover-conditioning, and further any time lag between the exit from theimmersion bath and entrance into a quench or rinse water vessel willcontribute to further over conditioning, events that often occur asfunctions of line geometry and strip line speeds. Some prior artimmersion systems attempt to remedy this problem through enhancingradiant or forced cooling, for example through the use of fans; however,such efforts not only result in uneven cooling or present safety hazardsfrom spraying molten salt droplets off on the conditioned metal andabout the immediate area, and are generally insufficient in avoidingover-conditioning as the metal object simply cannot be quenched orcooled fast enough after optimal conditioning is reached through animmersion process, as a comparison of the profiles 42, 44, 46 and 48 ofFIG. 2 to the respective profiles 62, 64, 66 and 68 of FIG. 4 clearlyillustrates.

The final treatment temperature of the heated, wetted metal object isdependent upon the object material, finish and dimensions, as well as onthe alkaline aqueous liquid properties (water content percentage, etc.).Generally, the final treatment temperature achieved is the melting pointof the conditioning salts within the chemical mixture plus an additionalvalue to effect conditioning of the oxide scale on the metal surface ata reasonable but not excessive rate. The time and heat required toeffect conditioning is dependent upon the thickness and material contentof the strip to be conditioned, which in some cases may act like a heatsink in absorbing heat that would otherwise raise the temperature of themetal surface and conditioning solution disposed thereupon. For example,in the case of one eutectic NaOH/KOH salt solution useful according tothe present invention, bringing the solution to about 170° C. (338° F.)is sufficient to melt the salt, but satisfactory conditioning of a thinmetal strip surface requires the temperature to be brought above thatpoint to about 232° C. (450° F.) with conditioning occurring virtuallyimmediately upon reaching said temperature, and whereas another stripexample having a greater thickness must be brought up to a highertemperature of about 288° C. to 315° C. (550° F. to 600° F.) and furtherheld there for a few seconds in order to result in an acceptableconditioning of the steel surface.

Accordingly, some processing system embodiments according to the presentinvention consider object material, finish and dimension and aqueousalkaline liquid properties. Other parameters useful in solving orachieving specified or desired object surface final treatmenttemperatures, and in some examples, in solving for final treatmenttemperature periods, may also be apparent to one skilled in the art.More particularly, for applications in which excess descaling chemicalsmay remain upon a treated surface after a specified or desired amount ofoxide conditioning has been achieved, the metal surface may be quicklybrought down below the final treatment temperature shortly afterachieving said final treatment temperature, in some embodiments withinthree seconds or less, thus preventing over-conditioning by theremaining reactants prior to rinsing. Illustrative but not exhaustiveexamples of such factors include process line observations and events,required metallurgical properties of a metal object in view of times andtemperatures in an annealing furnace (which in turn may dictate linespeed), object heat up rates, temperature hold times and dominant annealline speed requirements. Thus, processing system embodiments enableon-the-fly optimization in response to and subordinate to annealing andother line functions line changes to vary heat-up temperatures, holdtimes and alkaline aqueous liquid compositions and application amountsand rates, capabilities not possible with a conventional large hotimmersion process.

Referring now to the drawings and for the present to FIG. 5, a somewhatdiagrammatic representation of a process or system 100 for scaleconditioning section according to this invention is shown. The lineprocess 100 has an uncoiler 102 adapted to support and uncoil a coil ofsteel 104 for removal of scale formed during annealing. The uncoiler 102uncoils the steel from the coil 104 as a strip of steel 106 which isdrawn through a conditioning solution applicator 108 configured to applya thin coating 110 of an alkaline aqueous liquid according to thepresent invention and described above (e.g. comprising an alkali metalhydroxide or a mixture of alkali metal hydroxides and a surfactant) tothe top and bottom surfaces 112 of the steel strip 106. At variouspoints in the system 100, the uncoiled strip 106 is drawn through andguided by a set of conventional tracking and bridle rolls 107 configuredto keep the strip 106 on track and maintain proper tension in the strip106. While the diagram illustrates the line in a horizontal plane, it isnot the intention to limit the line configuration to a single plane.Certain elements such as the solution applicator 108 may be easilyconfigured in a vertical plane followed by other vertical or horizontalor angled elements as necessary to carry out the process and/oraccommodate physical line constraints. In some embodiments, thesystem/process 100 is a continuous anneal and pickle line, wherein theuncoiling element 102 also provides pre-heat and annealing furnaceelements in order to heat and/or anneal the steel strip as will beappreciated by one skilled in the art in the art. While FIG. 5illustrates a metal strip 106 moving relative to a stationaryapplication nozzle 108, other configurations where metal shapes otherthan strip may benefit from a movable application device relative to astationary metal object are also anticipated.

The surface temperatures of the steel strip surfaces 112 at applicationof the thin coating 110 of the alkaline aqueous liquid by theconditioning solution applicator 108 are below the Leidenfrosttemperature of the alkaline aqueous liquid, and in some embodiments alsobelow the melting point of alkali metal hydroxides within theconditioning solution. Sensors 105/115/116 may be provided comprisingtemperature-sensing devices (e.g. an infrared temperature sensor, acontact thermocouple, etc.) configured to measure temperatures of thestrip 106 at various points in the process/system 100 as needed toverify that a desired temperature has been achieved, thus at 105 priorto solution application by the conditioning solution applicator 108.Ambient environmental temperatures are generally below boiling point andLeidenfrost temperatures, and thus steel strip 106 uncoiled by anuncoiler 102 without annealing furnace elements or processes willtypically be at a temperature appropriate for application of thealkaline aqueous liquid by the conditioning solution applicator 108.

If, however, the uncoiler 102 anneals the strip 106, then the annealedstrip 106 must first be quenched or otherwise cooled to bring the stripsurfaces 112 down to a temperature below the Leidenfrost temperatureprior to application by the conditioning solution applicator 108. Insome applications, the line of steel strip 106 may be stopped or acooling time period must otherwise lapse until the strip surface 112temperatures cool to an acceptable temperature at the application of thesolution. In other examples, the system/process 100 may furtherincorporate a temperature cooling section at 105 which includes one ormore variable speed fans, flow control dampers, vents, or the like inorder to cool the strip surfaces 112 to a desired temperature asconfirmed by said temperature sensor 105.

Setting or achieving the final treatment temperature may also considerdifficulties in uniformly controlling the temperature of the metal stripsurfaces 112. For example, when the component 102 anneals the strip 106,such as within a continuous anneal and pickle line, the strip exiting anannealing furnace element 102 or an air cooler 105 positioned thereaftermay have temperature differentials between different regions, forexample between different edge regions, and/or between the top surfaceand the bottom surfaces. Such differential values may range from 100° F.(38° C.) to 200° F. (93° C.), depending on the strip dimensions (gage,width, thickness) and the metal composition (carbon, stainless, etc.).Thus, while some regions may be at a desired final treatmenttemperature, other regions of the strip 106 may be too hot andexperience Leidenfrost effects at application, or they may be too coldand thus not successfully brought up to the final treatment temperaturein the heating element 113 and thereby experience incompleteconditioning. (Such concerns are generally not an issue in traditionalimmersion salt bath, as the elevated temperatures of the molten saltbaths are applied long enough to result in uniform strip temperatures.)

Accordingly, in some embodiments, the cooling elements 105 cool thestrip surfaces 112 to a point below the Leidenfrost temperature of theconditioning solution plus an additional cooling margin value (forexample, 100° F. (38° C.) to 200° F. (93° C)). in order to ensure thatno regions of the strip surfaces 112 are above the Leidenfrosttemperature. Further, some heating elements 113 heat the strip surfaces112 to the final treatment temperature plus an additional heating marginvalue (for example, 100° F. (38° C.) to 200° F. (93° C)). in order toensure that all regions of the strip surfaces 112 are brought to thefinal treatment temperature. Additional cooling or heating marginsprovided to account for such regional differentials may be small or evenomitted for some very light gage steel strips, as their regionaldifferentials may be small or negligible, and in one aspect due to lowerheat sink and heat retention characteristics for lighter gage strips.

Formation of the thin alkaline aqueous liquid coating 111 by theconditioning solution applicator 108 may be achieved by a variety ofways, i.e., through any method or system that forms a uniform coating orcomplete wetting of the strip surfaces 112 with the conditioningsolution 110. Illustrative but not exhaustive examples of conditioningsolution applicator 108 elements and apparatuses include dunker rolleror roll/roller coaters 109 as well as spray nozzles, curtain coaters andapplicators, immersion methods and systems or combinations thereof.Solution metering or flow control articles may be utilized but are notgenerally required, and a conditioning solution applicator 108 may needonly incorporate simple application limiting means that ensures completewetting of the strip surfaces 112 to a specified maximum thicknessamount 111; for example, an air knife or wiper roller may be provided toremove excess conditioning solution 110 and effect the specified minimumand/or maximum solution thickness values 111, with excess solutionremoved and recovered for subsequent re-use. Other methods and systemsappropriate for use in assuring adequate and/or limited total thicknessvalues 111 of the conditioning solution 110 applied to the stripsurfaces 112 will also be appreciated by one skilled in the art.

The coated strip 106 is then driven into a heating section 113 whereinthe coated strip surfaces 112 are brought up to a specified finaltreatment temperature or temperature range above the melting point ofthe solution alkali metal hydroxide(s) in anhydrous form plus anadditional value to effect conditioning of the oxide scale on the metalsurface at a reasonable but not excessive rate. The specified finaltreatment temperature need be maintained only long enough to thoroughlycondition the engaged oxide scale, in some embodiments for no more or noless than a specified time period as described above, and wherein at theend of said period the heated strip temperature may be reduced below theconditioning temperature or range of temperatures by cooling orquenching in a cooling/rinsing section 114, which may also generallyrinse off any excess, non-consumed conditioning solution alkalineproducts.

As also described above, the specified final temperatures/ranges andtime periods are selected to produce preferred scale conditioning of thestrip 106, and more specifically to be sufficient in both temperatureand length of time to complete scale conditioning of the strip surfaces112, yet limited in either or both of length of time and hightemperature values in order to prevent over-conditioning of the stripsurfaces 112. In some embodiments, the desired level of conditioning isa specified level of least-oxide-to-pickle conditioning and minimal basemetal effect level selected as a function of strip material anddimension parameters, thereby minimizing the thickness and extent ofoxide scale formation while successfully conditioning the steel surface.

The mechanism of conditioning according to the present invention isbelieved to be generally comparable to that of conventional moltenoxidizing baths; the metal oxide is converted to a higher oxidationstate that is partially dissolved in the conditioning salts andsubsequent water rinse, the remainder rendered more readily removable byacid pickling. However, conditioning of the metal surface in the presentinvention occurs as the metal surface when completely wet with thesolution is then heated until the water is evaporated and the salts aremelted and react with the oxide on the strip surface, which occursrapidly, often within seconds. In some embodiments, the conditioningprocess is terminated by rinsing and thereby cooling or quenching thestrip 106 in the rinsing station 114 by the end of a specifiedconditioning time period (i.e., after effective conditioning occurs andprior to the occurrence of excessive oxide formation by remaining,residual reactants) to bring the strip surface 112 temperatures downbelow conditioning temperatures and also rinse the conditioning solution110 off of the strip surfaces 112, for example through an array of waterspray nozzles (not shown) being supplied with water, sometimes throughuse of a pump from a collection sump located below a spray area 114, andstill other rinsing station 114 systems and methods will be appreciatedby one skilled in the art. Temperature sensing/cooling elements 115interposed between the heating section 113 and rinsing section 114 mayensure that the strip surface 112 temperatures are quenched belowconditioning temperatures.

After the rinsing station 114, the conditioned strip 118 may be drivento an acid pickling section 120. By minimizing the degree of oxideformation, the present invention correspondingly reduces the amount ofsubsequent surface pickling required, which may thus reduce associatedpickling processes as well as reduce surface dulling and roughening dueto pickling relative to prior art processes. In contrast, prior artimmersion bath processes using molten oxidizing baths may over-conditionscale on a strip and form a cohesive base metal-iron oxide interfaciallayer and subjacent chromium depleted zone, thus requiring moreaggressive pickling and increasing both pickling costs and pickledsurface roughness while also reducing product yield due to increasedmetal removal.

Acid pickling in the process 100 at 120 usually includes one or moreacid tanks containing sulfuric acid and/or a mixture of nitric,hydrofluoric or other acids for submersion of the strip 106, althoughacid spray could also be used. In some embodiments, multiple acidpickles are utilized, and one or more of such pickle tanks may be usedas required on any given strip 110 of stainless steel depending on manyfactors, including the composition of the steel, the thickness of theoxide, and other factors known in the art. Novel pickling compositionsincorporating organic acids such as citric acid, or moreenvironmentally-attractive acid mixtures incorporating oxidizing agentssuch as peroxides, would also experience enhanced performance if used inconjunction with this invention. With the scale removal process completeafter pickling and rinsing, the descaled strip 122 is ready to berecoiled into a finished steel coil 124 on a recoiler 126.

Surface analyzers are generally provided within the system/process 100,for example within or adjacent to the rinsing station 114, thetemperature sensing/cooling elements 105/115/116, within the picklingsection 120, etc., said surface analyzers configured to monitor thestrip surfaces 112 to detect and/or determine the amount of scale formedon the surface of the strip to be conditioned, a lack of conditioning orover-conditioning of the conditioned strip, etc., and to otherwiseprovide feedback and monitoring of conditioning performance of thesystem/process 100 to one or more line dynamics operating components,systems or other management elements 128. The line dynamic systemcomponent 128 may also communicate with various temperaturesensing/cooling elements provided throughout the system/process 100 (forexample, at temperature sensing/cooling elements 105/115/116, theconditioning solution applicator 108, the heating section 113, therinsing station 114 and/or the acid pickling section 120, etc.), therebyensuring specified strip temperatures during application of theconditioning solution, conditioning, rinsing and pickling operations aswell as to enable optimizing of performance of the system 100 inresponse to said temperature observations. Inputs to the line dynamicsoperating system 128 may also include present system conditions, such asambient temperatures, conditioning solution storage tank level sensors,flow controllers and distribution sensors, and storage tank temperaturesensors. In some embodiments, chromium concentrations are observed inthe rinse water in the rinsing station 114, thereby providing a directmeasure of chromium removed from the scale to the line dynamic systemcomponent 128 or other operator, and other inputs will be appreciated byone skilled in the art.

The line dynamics operating system 128 may receive strip variables suchas strip material composition, gauge, width, and any other specialprocessing information as discussed above and elsewhere herein and thenresponsively determine specified conditioning temperatures and timeperiods, or to otherwise control a conditioning schedule or other system100 parameters for the particular strip of steel 106 being treated. Insome embodiments, the line dynamics operating system 128 includes acomputer in communication with a memory in which is stored time,temperature and other parameters required for conditioning each of aplurality of types of steel, and further based on composition, gauge,width, line dynamics, etc.

Said surface analyzer elements may continuously monitor the condition ofthe strip 106 and if the strip surface 112 condition falls outsidepredetermined parameters, the line dynamics operating system 128 mayadjust system/process 100 parameters to bring monitored surfaceconditions back within the required performance thresholds. Illustrativebut not exhaustive parameters that may be controlled by the linedynamics operating system 128 include an amount of energy or heatexpended and directed toward the strip 106 by, or amount of temperatureincrease effected by, the uncoiling/annealing/preheating element 102 andthe heating section 113; the motive speed of the strip 106 relative toany of the system elements 108/113/115/114/116/120; the amount ofcooling air, temperature or amount of temperature increase effected bycooling elements at 114, 115 or 116; and other system 100 parameters maybe controlled by the line dynamics operating system 128.

The present invention enables on-the-fly scale conditioning optimizationby varying one or more of (1) terminal conditioning temperatures orranges of temperatures, (2) chemical composition components, (3) amountsof reactants utilized (e.g. amounts of chemicals applied to the scale onthe strip surface 106 at 108) and (4) reaction time periods or ranges(e.g., by variably cooling the strip at a desired point to quench out aconditioning reaction). Prior art salt bath immersion processes cannotachieve such objectives due to the thermal inertia of the large saltmass in the salt baths, the line-speed dependent exposure times to thechemicals, and the static chemical composition of said baths.

Optimal amounts of scale conditioning may also be defined with respectto least- oxide-to-pickle condition specifications or observations forgiven metal object material and dimension parameters, as well as withrespect to minimal base metal effects. In one aspect, the appearance andcost of producing a final pickled surface at 120 may provide a measureof the value of changing scale conditioning variables at any of thevarious process/system 100 elements. For example, in some embodiments,optimal conditioning may comprise determining that no chrome oxide andonly minimal or trace amounts of iron oxide have been formed on a 3XX or304 stainless steel strip surface, wherein observing a heavier ironoxide or a nickel oxide formation would indicate that a conditioningsalt in a solution applied at 108 has stayed in contact with the stripsurface for too long or at too high of a treatment temperature. Theconditioning time period could then be shortened by earlier rinsing orquenching at 114, or the extent and/or rate of conditioning could belowered by lowering the strip conditioning temperatures achieved at 113,in one aspect thereby preventing dulling of the strip surface andreducing the amount of pickling required at 120 to achieve a finishedsurface. Depending on the specific alloy being processed and its uniquescale composition reactivity, it is also conceivable that the chemicalcomposition applied to the scale for conditioning purposes be adjustedto provide more or less oxide reactivity in concert with or independentof controlling conditioning temperature and time.

In general, conditioning occurs more rapidly on metal strips with goodtight surface conditions and thinner gages, and the extent of theconditioning may be fine tuned with a greater tolerance or accuracythrough adjustment of time and temperature parameters relative to dullermetal objects and those with heavier strip materials, the processes andsystems according to the present invention thus providing opportunitiesfor energy savings and enhanced performances and efficiencies inconditioning said thinner gage/good tight surface objects over prior artsubmersion systems. The descaling system can also react to the varyingabsorption and emissivity of shiny versus dull metal surfaces.

By keeping the temperatures of the steel coil or strip surfaces belowthe boiling point and the Leidenfrost temperature or Leidenfrost pointof the conditioning solution during application of said solution,problems with respect to the Leidenfrost effect are avoided. The“Leidenfrost effect” with respect to a metal strip is a mottled orspeckled surface appearance of the strip which reveals patches, or spotsof incomplete scale conditioning, and which is believed to occur due tothe Leidenfrost effect on an aqueous solution of chemicals if thesurface temperature of a strip during application is above what is knownas the Leidenfrost temperature or Leidenfrost point of the conditioningsolution. If the strip is above the Leidenfrost temperature of theconditioning solution when the conditioning solution is applied (whichis typically at or above the boiling point of aqueous conditioningsolution), then a thin film of the solution is converted to a vaporphase barrier between the metal strip surface and the applied solution,this vapor phase barrier preventing the conditioning solution fromcontacting the surface of the strip and depositing conditioningchemicals on the metal surface upon evaporation of the liquid, resultingin a failure to condition portions of the surface and thereby producinga mottled appearance due to the contrast between conditioned andunconditioned areas. Thus, as used herein, the term “a temperature belowwhich the Leidenfrost effect appears” refers to a temperature at whichno appreciable scale in the form of dark spots exists after scaleconditioning according to this invention and subsequent pickling. TheLeidenfrost effect is well known and described in many publications. Theinterested reader is referred to U.S. Pat. No. 6,450,183 issued to Cole,et al. for “Composition, apparatus, and method of conditioning scale ona metal surface” on Sep. 17, 2002, as well as to two other publications:“Disk Model of the Dynamic Leidenfrost Phenomenon” (Martin Rein at DFD96meeting of American Physical Society) and “Miracle Mongers and TheirMethods” (pages 122-124 by Harry Houdini, published 1920 by E. P.Dutton).

Some embodiments of the present invention avoid the Leidenfrost effectby first completely wetting the strip surface to be descaled with anaqueous alkali metal hydroxide(s) conditioning solution to form awetting layer when the metal surface temperatures are at a temperaturebelow the Leidenfrost temperature of the conditioning solution, and thensubsequently heating and increasing the wetting layer solution (e.g.111) and the surface temperatures of the strip surface (e.g. 112) to atemperature above the melting point of the essentially anhydrous form ofthe alkali metal hydroxide material in the conditioning solution plusthe additional value described above to reach the final treatmenttemperature for a sufficient time to thereby condition the metal stripsurface (e.g., 112). As used herein, the term “essentially anhydrousform of the material” means after the water of solution is evaporated,even though there may be some water of hydration still present in thematerial. In this fashion, the formation of the vapor barriers known tocause the Leidenfrost effect, and thus the Leidenfrost effect upon thesurface of the conditioned strip surfaces (e.g., 118/122) is avoided.

The boiling and Leidenfrost temperature points of an aqueous causticconditioning solution are a function of caustic concentration. Forexample, for conditioning solution embodiments with low (40% by weightor lower) alkali hydroxide concentrations, and wherein application isdesired at surface temperatures below both boiling and Leidenfrosttemperature points, the steel strip surface 112 or coil 104 temperaturesat application of the conditioning solution range should not exceed fromabout 180° F. (82° C). to about 260° F. (127° C.), though for solutionswith higher (47% or higher) alkali hydroxide concentrations the steelstrip surface 112 or coil 104 may be at higher temperatures duringapplication, meeting or even exceeding 290° F. (143° C.).

Additional energy savings may be realized by applying the aqueouscaustic conditioning solution while a steel coil or strip is at higherelevated temperatures, temperatures above the conditioning solutionboiling temperatures, as well as above a salt fusion temperature pointof the solutions, but still near and below the Leidenfrost temperatureof the conditioning solution. Thus, in one example where an applicationtemperature of 400° F. (204° C.) is above both the boiling point and thesalt fusion point of a conditioning solution, a relatively smaller heatincrease is required by a furnace or heating station 113 to get to afinal treatment temperature ranging from 550° F. (288° C.) or 600° F.(316° C.)). Thus, less heat energy is expended by a furnace or heatingstation 113, and/or over shorter the time periods, relative to therequirements for heat energy expenditures when a lower applicationtemperature is selected (for example, one below the boiling point and/orthe salt fusion point).

Different compositions may be used to effect descaling according to thisinvention. In one embodiment, a eutectic of sodium hydroxide (NaOH) andpotassium hydroxide (KOH) at about 42% sodium hydroxide and about 58%potassium hydroxide a base alkali hydroxide composition is provided.This is a low melting composition in its essentially anhydrous condition(170° C., 338° F.), and when the water of the aqueous solution isevaporated and the remaining hydroxide fused, it is effective to performscale conditioning. Other materials may also be added to the solution tomodify the properties of either the solution or the composition, and forexamples and other information commonly-assigned U.S. Pat. No. 6,450,183issued to Cole, et al. on Sep. 17, 2002 and entitled “Composition,apparatus, and method of conditioning scale on a metal surface,” theentire disclosure of which is hereby incorporated by reference, andwhich provides that additives such as potassium carbonate, potassiumchlorate, sodium nitrate, sodium permanganate, and potassiumpermanganate are beneficial, for example at from about one weightpercent (1%) to about five weight percent (5%).

Descaling performance and costs are directly related to the percentagesof base alkali hydroxide compositions within conditioning solutionsaccording to the present invention. Although scale conditioning may begenerally achieved over a wide range of alkali hydroxide concentrations(for example, from about 5% to about 65% by weight), differentpercentage values have different impacts on system performance. Moredilute/low alkali hydroxide percentages (from about 5% to less thanabout 20%) have proportionately lower surface tension and improvedsurface wetting characteristics relative to higher concentrations, butalso impose proportionally greater heating energy penalties in order toheat and evaporate the proportionately larger amounts of water in thesolution to effect scale conditioning by the fused hydroxide(s); theyalso require that the incoming strip temperature be lower to avoidLeidenfrost (or optionally, also the boiling point) of the more dilutesolution. Higher concentration solutions (from about 20% to less thanabout 50%) appear to strike a good balance between delivering reasonabledissolved solids content at acceptable energy requirements. U.S. Pat.No. 6,450,183 further provides that as the concentration of the salt insolution increases, the upper temperature that can be used withoutencountering Leidenfrost effect increases, for example to about 700° F.For example, a 47% solution disposed upon a metal surface and thenheated requires two British Thermal Units (BTU's) of energy per appliedwet gram to heat the solution enough to bring the contained alkalihydroxide(s) into a molten or fused state on the surface of the metal.While not insignificant, the heat input requirement for dehydrating andfusing the applied chemical represents only about 10% of the total heatenergy required by the process; the other 90% or so of the energy isabsorbed by the metal itself as its temperature is raised fromessentially ambient to about 600° F. (e.g., of the heating section 113).

Though it may initially appear to be preferable to use higher causticconcentrations within the conditioning solution (e.g. 48% or more),higher alkali hydroxide concentration conditioning solutions presentother problems and difficulties. At concentrations much above about 47weight percent, supersaturation and crystallization of some of thedissolved chemicals occurs at ambient temperatures. This requiresmixing, transport, holding tanks, and application apparatus all to beheated to maintain homogeneous solutions. Besides presenting highermanufacture, transportation, storage, and delivery difficulties andassociated costs, the higher-solids concentration in such solutionspresents problems in achieving complete wetting of steel surfacesrelative to lower concentration solutions. Higher surface tensions causeincomplete surface coating and wetting of the metal surface to beconditioned, in particular during dehydration and fusion of the alkalihydroxide during heat-up (e.g. by the heating section 113), which maycause incomplete scale conditioning. Wet-out and flowabilitycharacteristics are also a function of metal surface attributes, andthese problems are more pronounced in conditioning the smooth, minorfinishes of the more expensive stainless and superalloy steel strips.Any area that is poorly conditioned will not be completely pickled(e.g., by the acid pickling section 120), and even a 0.01% failure intotal area surface conditioning may be unacceptable, particularly in thecase of stainless steel. In order to facilitate the use of higherconcentration caustic solutions, e.g., those of about 25% to near 50%,as discussed above, in some embodiments of the present invention a basealkali hydroxide composition of about 42% sodium hydroxide and about 58%potassium hydroxide incorporates a small amount of surfactant into thesolution, which reduces the solution surface tension caused by thepresence of relatively higher dissolved caustic solids percentages inthe solution, while also enabling the conditioning solution to exhibitlow foam characteristics.

In another aspect, scale conditioning according to the present inventiontakes place in the presence of oxidizers, generally in anoxygen-containing atmosphere. Although the alkaline aqueous liquid neednot contain any oxidizing agents, the thin film 111 will have anoxidizing effect on the surface oxides and thereby convert them to thedesired higher oxidation state due to the absorption of atmosphericoxygen by the wetting solution and/or the diffusion of atmosphericoxygen through the molten salt film 111, and wherein the heating section113 heats the wetted strip in an oxygen-containing atmosphere. In oneexample, the surface of a 10.16 centimeter (4 inch)×15.24 centimeter (6inch) panel of 0.635 millimeter (0.025 inch) gage type 304 annealedstainless steel type 304 (18/8 chrome-nickel) was wet with an alkalineaqueous liquid solution comprising a surfactant and a base alkalihydroxide composition comprising a eutectic of about 42% sodiumhydroxide (NaOH) and about 58% potassium hydroxide (KOH) and heated in afurnace in an oxygen-containing, ambient air atmosphere at about 399° C.(700° F.) for about 90 seconds, bringing the surface temperature of thewetted steel to a final treatment temperature of about 232° C. (450°F.). Upon cooling of the panel, a visual examination revealed that apronounced scale conversion had taken place and, after a subsequentwater rinse, a well-conditioned surface was apparent. Pickling of thepanel after the water rinse confirmed the visual assessment of success.

One skilled in the art must also appreciate the potentially competingreactions between the alkali hydroxides and atmospheric oxygen tocomplete a desired descaling reaction with the simultaneousneutralization reaction between atmospheric carbon dioxide and causticalkalis to form ineffective alkali carbonates. If a heat up rate is slowand/or the atmosphere to which the coated metal is exposed to is high incarbon dioxide (as would be the case in a furnace heated with acarbon-based fuel such as natural gas or propane to produce carbondioxide as a product of combustion), the desired scale conditioningreaction could be retarded or prevented altogether.

One benefit of the present invention is the ability to utilizeconditioning compositions that cannot be used effectively inconventional anhydrous molten salt baths because the mass of materialsurrounding the surface prevents atmospheric oxygen diffusion. Thesolution can also utilize additives that may be unstable at typicalanhydrous molten salt bath temperatures. Furthermore, this inventioneliminates the presence of reaction products in the applied salt andthus allows complete control of the chemistry of the salt at the metalsurface. Further, with respect to direct surface wetting embodiments ofthe present invention, the quantity of salt consumed can be controlledthrough to a specified amount, in contrast to immersion systems, whereinsalt consumption is largely dictated by the quantity of salt thatadheres to the surface of the metal as it is withdrawn from the moltenbath. Additionally, as in some cases it may be desirable to use adifferent salt chemistry when different metals are treated, switchingsolutions is easily accomplished, with no need to heat large baths ofeach solution. Conventional immersion technology requires a molten saltbath or tank that holds tens of thousands of pounds of liquid, hotchemical. Molten salts in general are excellent heat storage media andrequire significant time (several hours or longer) to cool or raisetheir temperature. This severely limits the ability to change processtemperature “on the fly” and prevents real-time descaling optimizationfrom being practicable. Taking into account the now-available processvariables of variable chemical compositions, application rates, reactiontimes, and reaction temperatures, it is now possible for the first timeto fully optimize precise descaling performance dynamically.

It should be noted that while the embodiments discussed thus far usesodium or potassium cations within the alkaline aqueous liquidconditioning solution, alternative solutions may utilize differentcations, and associated descaling parameters and effects are primarilydependent upon the particular anion present. In one aspect, alternativeconditioning solution compositions may work about as effectively withone cation as with another if other factors, such as solubility andcompatibility, are equal. For example, sodium nitrate or potassiumnitrate may also be effective in conditioning solutions according to thepresent invention, and may give comparable results in general, thoughtypically much less soluble in a base composition and thus perhapsrequiring different relative cation and/or surfactant concentrations.Other examples provided by U.S. Pat. No. 6,450,183 include sodiumbisulfate, sodium carbonate, potassium carbonate, sodium formate, sodiummetasilicate, sodium nitrite, sodium acid pyro phosphate and mono sodiumphosphate. In some cases, the selection of a cation of an additive orcaustic compound utilized may be dictated by availability. It is alsonoted that use of a surfactant is dependent upon compatibility withsolution additives or base cations: for example, surfactants may beincompatible with permanganate compounds and thus excluded from suchembodiments.

Performance of alternative compounds used as sole descaling agents maybe easy to judge visually, wherein ineffectiveness of conditioning maybe confirmed by subsequent pickling after which an original scale wouldbe present in unchanged form. Evaluation criteria for selectingappropriate conditioning solutions and specified time and temperaturesmay include appearance of conditioned oxide with regard, e.g., to color,opacity, and uniformity; ease of removal of conditioned oxide byrinsing, wiping or subsequent acid pickling, and final appearance of adescaled metal surface with regard, e.g., to color, brightness,uniformity, and freedom from residual oxide. It is to be understood thatthese several criteria can vary independently in degree and directionone from another, so that there is a certain subjective element to thequantitative assignment of detrimental or beneficial effects of anydescaling agents or additives.

While the present invention has been illustrated by the description ofthe embodiments thereof, and while these embodiments have been describedin considerable detail, it is not the intention to restrict or in anyway limit the scope of the appended claims to such detail. Additionaladvantages and modifications may readily appear to those skilled in theart. Therefore, the invention, in its broadest aspects, is not limitedto the specific details, the representative apparatus, or theillustrative examples shown and described. Accordingly, departures maybe made from such details without departing from the spirit or scope ofthe applicants' general inventive concept.

Units which are used in this specification and which are not inaccordance with the metric system may be converted to the metric systemwith the aid of the following formulas: 1° C.=(° F.−32) 5/9; 1inch=2.54×10⁻² m; and 1 F.p.m. (foot per minute)=5.08×10⁻³ m/sec.

1. A system for treating oxide scale on the surface of a metal object,comprising: a temperature control apparatus that controls thetemperature of metal object's surface to an application temperaturebelow the Leidenfrost temperature point of an aqueous conditioningsolution comprising an alkali metal hydroxide, the metal object'ssurface comprising an oxide scale having an initial depth from the metalobject's surface; an application apparatus that wets the metal object'ssurface at the controlled temperature with a thin layer of the aqueousconditioning solution, the aqueous conditioning solution thin layerengaging the oxide scale; and a heating apparatus that heats the wettedmetal object surface to a final conditioning temperature that is above amelting point of the alkali metal hydroxide in an anhydrous form by anadditional value selected to effect conditioning of the oxide scale onthe metal surface at a reasonable but not excessive rate, the wettedmetal object surface heated to the final conditioning temperatureevaporating water in the aqueous conditioning solution and melting thealkali metal hydroxide in the anhydrous form on the metal object'ssurface, the melting alkali metal hydroxide reacting with the engagedoxide scale and reducing the oxide scale to a conditioned depth from themetal object's surface that is less than the initial depth; wherein thesystem terminates additional conditioning of the metal object's surfacebeyond the conditioned depth, the terminating preventing a creation ofan additional oxide scale beyond the conditioned depth from the metalobject's surface.
 2. The system of claim 1, wherein the systemterminates the additional conditioning of the metal object's surfacebeyond the conditioned depth by the application apparatus applying theaqueous conditioning solution thin layer in an amount wherein thereacting of the melting alkali metal hydroxide with the engaged oxidescale consumes enough of the melting alkali metal hydroxide in the thinlayer on the metal object's surface to prevent additional conditioningof the metal object's surface.
 3. The system of claim 1, furthercomprising: cooling apparatus that quenches the metal object's surfaceto a temperature below the final conditioning temperature at an end of aconditioning time period that is selected to terminate the additionalconditioning as a function of a material parameter and a dimensionparameter of the metal object.
 4. (canceled)
 5. The system of claim 3,wherein the final conditioning temperature and the conditioning timeperiod are selected to produce a specified extent of conditioning of thescale on the metal object's surface.
 6. The system of claim 5, whereinthe additional value to effect conditioning of the oxide scale isselected from a range of zero to about 200° F. (94° C.). 7-11.(canceled)
 12. The system of claim 6 wherein the aqueous conditioningsolution comprises: a eutectic blend of sodium and potassium hydroxidesat about 30% by weight; about 3% by weight sodium nitrate; about 67% byweight water; and less than about 1% by weight of at least one nonionicsurfactant.
 13. The system of claim 12 wherein the eutectic blendcomprises about 18% by weight potassium hydroxide and about 12% byweight sodium hydroxide. 14-16. (canceled)
 17. The system of claim 6,wherein the final conditioning temperature is an increase over theapplication temperature ranging from about 150° F. (65° C.) to about200° F. (94° C.)).
 18. A method of treating oxide scale on the surfaceof a metal object, comprising: controlling a temperature of a metalobject's surface to an application temperature below the Leidenfrosttemperature point of an aqueous conditioning solution comprising analkali metal hydroxide, the metal object's surface comprising an oxidescale having an initial depth from the metal object's surface; wettingthe metal object's surface at the controlled temperature with a thinlayer of the aqueous conditioning solution, the aqueous conditioningsolution thin layer engaging the oxide scale; heating the wetted metalobject surface to a final conditioning temperature that is above amelting point of the alkali metal hydroxide in an anhydrous form by anadditional value selected to effect conditioning of the oxide scale onthe metal surface at a reasonable but not excessive rate, therebyevaporating water in the aqueous conditioning solution and melting thealkali metal hydroxide in the anhydrous form on the metal object'ssurface at the final conditioning temperature and causing the meltingalkali metal hydroxide to react with the engaged oxide scale and reducethe oxide scale to a conditioned depth from the metal object's surfacethat is less than the initial depth; and terminating additionalconditioning of the metal object's surface beyond the conditioned depth,the terminating preventing a creation of an additional oxide scalebeyond the conditioned depth from the metal object's surface.
 19. Themethod of claim 18, wherein the terminating the additional conditioningcomprises the reacting of the melting alkali metal hydroxide with theengaged oxide scale consuming enough of the melting alkali metalhydroxide on the metal object's surface to prevent additionalconditioning of the metal object's surface.
 20. The method of claim 18,further comprising: selecting a conditioning time period to terminatethe additional conditioning as a function of a material parameter and adimension parameter of the metal object; and wherein the terminating theadditional conditioning comprises quenching the metal object's surfaceto a temperature below the final conditioning temperature at an end ofthe conditioning time period.
 21. The method of claim 20 wherein thequenching comprises rinsing the alkali metal hydroxide from the metalobject's surface.
 22. The method of claim 20, further comprisingselecting the final conditioning temperature and the conditioning timeperiod to produce a specified extent of conditioning of the scale on themetal object's surface.
 23. The method of claim 22, further comprising:selecting the additional value to effect conditioning of the oxide scalefrom a range of zero to about 200° F. (94° C.).
 24. The method of claim22, further comprising optimizing the conditioned depth by varying atleast one of the final conditioning temperature, components of theaqueous conditioning solution, relative amounts of reactants utilized inthe aqueous conditioning solution and the conditioning time period as afunction of the material parameter and the dimension parameter.
 25. Themethod of claim 22, wherein the specified extent of conditioning is aleast-oxide-to-pickle level of conditioning of scale on the metalobject's surface and a minimal base metal effect, the method furthercomprising: selecting the final conditioning temperature and theconditioning time period as a function of an amount of scale on themetal object's surface.
 26. The method of claim 22 wherein theconditioning time period is no more than about thirty seconds.
 27. Themethod of claim 26 wherein the conditioning time period is about threeseconds.
 28. The method of claim 22 wherein the heating the wetted metalobject surface comprises heating in an oxygen-containing atmosphere. 29.The method of claim 22 wherein the aqueous conditioning solutioncomprises: a eutectic blend of sodium and potassium hydroxides at about30% by weight; about 3% by weight sodium nitrate; about 67% by weightwater; and less than about 1% by weight of at least one nonionicsurfactant.
 30. The method of claim 29 wherein the eutectic blendcomprises about 18% by weight potassium hydroxide and about 12% byweight sodium hydroxide.
 31. The method of claim 22, further comprisingdetermining the Leidenfrost temperature point as a function of a causticconcentration of the aqueous conditioning solution.
 32. The method ofclaim 31, further comprising: selecting the application temperature nearthe Leidenfrost temperature point and above a boiling point of theconditioning solution.
 33. The method of claim 32, further comprisingselecting the application temperature above a salt fusion temperaturepoint of the conditioning solution.
 34. The method of claim 33, furthercomprising: selecting the final conditioning temperature as an increaseover the application temperature ranging from about 150° F. (65° C.) toabout 200° F. (94° C.)).